[MRG] improve planning to ask users for a dataset; other small fixes

[HuaizhengZhang]

Closes

• [bug] sometimes the generated training file would be .csv #29

• rename target_source;

• improve the log of mle start

• improve planning ability, ask users for dataset

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[HuaizhengZhang]

as more conflicts are coming, I decide to mrg first...